62 DEAN. 



mass is seen at the left; at the right the notochord has separated from the inner 

 layer ; the epidermic stratum of the outer layer is now clearly marked, e. 

 X about 100. 



Fig. 70. Transverse section of the hinder trunk region of an embryo of 43 

 hours, showing the relation of the inner layer to the chorda and mesoblast. 



Fig. 71. Sagittal section of anterior end of neuron of embryo of 11 primitive 

 segments, slightly older than that of Fig. 66, showing the point of connection of 

 the anterior end of the embryonic brain with the formative epiblast, *. The germ 

 layers are lettered as before ; the Deckschicht of the outer layer, d^ is now clearly 

 differentiated. 



Fig. 72. Transverse section through the point * of the preceding figure. 

 Lettering as before. 



Fig. "ji. Transverse section of region slightly posterior to that of Fig. 72. 



Fig. 74. Sagittal section of anterior end of neuron of embryo of 46 hours. 

 The lettering corresponds with that of Fig. 71. Between the limits x-x occur 

 points of union between brain wall and formative epiblast, o ; the anterior third 

 of this region is broadly fused with the outer layer. Fusion also occurs in front 

 of the embryonic brain between the formative epiblast and the cells of the inner 

 layer, /. 



Figures, with exception of Figs, i, 2, 3, 46-48, are from camera drawings. 

 Material was fixed either in picro-sulphuric or in corrosive glacial acetic. Haema- 

 calcium and Delafield's haematoxylin were the staining agents. 



